1. Field of the Invention
This invention relates to a bicycle deraileur of the type which comprises a parallelogrammic linkage mechanism and a chain guide mechanism supported thereby, wherein the linkage mechanism is forcibly deformed against the restoring force of a return spring by a control cable to laterally displace the chain guide mechanism for shifting one sprocket to another diametrically different sprocket. More particularly, the invention relates to such a deraileur which is capable of pre-loading or pre-torsioning the return spring even if the chain guide mechanism is locked in its lateral movement.
2. Description of the Prior Art
As is well known, a typical bicycle deraileur comprises a parallelogrammic linkage mechanism mounted to a suitable portion of a bicycle frame and a chain guide mechanism supported by the linkage mechanism. More specifically, the linkage mechanism includes a base member fixed to the bicycle frame, a pair of parallel links each pivotally pinned at one end to the base member, and a movable member pivotally pinned to the other end of each link. In the case of a rear deraileur, the chain guide mechanism is located adjacent to a multiple freewheel and comprises a spring-biased shift frame pivotally mounted to the movable member of the linkage mechanism and supporting a pair of pulleys for engagement with a chain. In the case of a front deraileur, on the other hand, the chain guide mechanism is positioned adjacent to a multiple chainwheel and comprises a pair of guide plates arranged on both sides of the chain as fixed on the movable member of the linkage mechanism.
The linkage mechanism is usually biased toward its normal position by a return spring. Typically, such spring is mounted on a pin connecting two adjacent parts (e.g. the base member and one of the links) of the linkage mechanism and has both ends engaging with these two parts.
The linkage mechanism is pivotally deformable by a control cable extending from a remote speed change lever for connection to one of the links. Thus, when the speed change lever is operated in one direction, the linkage mechanism is pivotally deformed against the elastic force of the return spring to displace the guide mechanism laterally of the freewheel (rear deraileur) or the chainwheel, thereby shifting the chain from one sprocket to another diametrically different sprocket. When the speed change lever is operated in the reverse direction, the torsioned return spring functions to bring the linkage mechanism toward its original position.
In the above prior art deraileur, the return spring is torsioned as much as the linkage mechanism is pivotally deformed. More specifically, if the two adjacent parts of the linkage mechanism engaging with the spring ends are pivoted by an angle of 45.degree. for example relative to each other upon pivotal deformation of the linkage mechanism, the return spring is also torsioned by an angle of 45.degree.. In other words, the elastic restoring force of the return spring increases in proportion to pivotal angle of the linkage mechanism, causing a large difference in restoring force of the spring between the normal position of the linkage mechanism and the maximally deformed position thereof. Such largely increasing returning force of the spring results in deteriorated operability of the speed change lever because the lever must be imparted a sufficient friction which resists a maximally increased returning force of the spring to prevent spontaneous pivotal movement of the lever.
It is conceivable to increase the number of helixes of the return spring for the purpose of reducing the variation in restoring force per unit pivotal deformation of the parallelogrammic linkage mechanism. However, such an attempt renders the spring very bulky, which causes troubles in assembly of the deraileur or otherwise requires enlargement of the deraileur with attendant weight increase.
Further, with the prior art deraileur, the control cable is connected directly to one of the links, so that the speed change lever is not operable independently of the linkage mechanism. This means that when the bicycle is not running, the chain engaging with or locked on one sprocket prevents the chain guide mechanism or the movable member of the linkage mechanism from moving laterally. Thus, it is impossible in such a locked state to operate the speed change lever.
Japanese Patent Publication No. 53-11742 (Published: Apr. 24, 1987; Application No.: 49-143965; Filed: Dec. 12, 1974; Applicant: Shimano Industrial Co., Ltd.; Inventor: Mitsuhide ISOBE) discloses a bicycle rear deraileur which is capable of pre-loading a spring-biased parallelogrammic linkage mechanism. More particularly, the linkage mechanism is provided with an operating member which is pivotally mounted on a pin which connects a base member to a first one of two parallel links. The operating member is always urged by a return spring in a pivotal direction to engage with the first link which in turn is urged by a weaker counteracting spring in an opposite pivotal direction. The operating member is connected to a control cable which, when tensioned by a remote speed change lever, causes the operating member to pivot away from the first link against the restoring force of the first spring.
With the deraileur of the above publication, when the speed change lever is operated to tension the control cable, the operating member starts moving away from the first link which, however, follows the operating member under the action of the counteracting spring, thereby pivotally deforming the linkage mechanism as a whole for speed change shifting. When the control cable is freed from tension, on the other hand, the restoring force of the return spring overcomes the elastic force of the weaker counteracting spring to bring the operating member and the first link to their respective original positions.
If the parallelogrammic linkage mechanism is locked for example due to non-running of the bicycle, the operating member alone can be pivoted by tensioning the control cable by means of the speed change lever. Such pre-pivoting of the operating member, i.e., pre-loading of the return spring, puts the locked linkage mechanism in a standby condition for shifting since the first link is always under the action of the counteracting spring. Thus, upon start of the bicycle, the linkage mechanism will be immediately deformed by the elastic force of the counteracting spring.
The deraileur of the Japanese publication, however, has a disadvantage of requiring two separate springs which complicates the overall structure and necessitates exact balancing the two springs.